Control of weeds is an important aspect of crop management. Due to several undesirable properties associated with the use of chemical herbicides, alternative weed control practices, including the use of biological herbicides, are desired. For example, rising economic, environmental and social costs associated with agricultural inputs, spray drift, pesticide residues, government legislation for reduced pesticide use, along with the development of herbicide resistance in weeds, make biocontrol agents attractive strategies for weed control.
Biological control of weeds with microorganisms (bioherbicides), preferably involves the production and application of a weed-specific pathogen to a target weed. The weed specific pathogen is typically a fungus or bacterial pathogen that inhibits or suppresses root, shoot or both root and shoot growth, development, or both growth and development, thereby reducing weed competition. The development of biological crop protection products (bioherbicides) for economically important weed problems in agricultural field crops may help to facilitate harvests, secure yields, and protect the environment. Biological control provides an additional tool to complement an integrated weed management system and helps sustainable agricultural systems by maintaining the ecosystem balance through the preservation of plant and microbial diversity in the field.
There are several documents disclosing the use of fungi as biocontrol agents. For example, U.S. Pat. No. 5,993,802 teaches methods for suppressing the growth of Calamagrostis canadensis using an isolate of a low temperature basidiomycete fungus, Coprinus psychromorbidus. U.S. Pat. No. 5,472,690 teaches of a mycoherbicide (including at least one or both of Fusarium nivalis and Colletotrichum calamagrostidis) effective in the control of Calamagrostis canadensis and/or related grasses. The control of crabgrass using fungi is disclosed in U.S. Pat. No. 5,952,264, using the fungus Cochliobolus intermedius, and U.S. Pat. No. 5,635,444 using a fungus selected from the genus Curvularia. U.S. Pat. No. 5,747,029, teaches the control of sicklepod weeds using the fungus Myrothecium verrucaria. The control of nutsedge weeds using the fungus Dactylaria higginsii is disclosed in WO 98/08389. U.S. Pat. No. 4,606,751 teaches the biocontrol of Johnson grass using Bipolaris sorghicola spores that are suspended in a solution of water and surfactant, and sprayed onto a field in which the weed is growing.
Annual grassy weeds such as Setaria viridis (L.) Beauv. (commonly known as green foxtail, pigeongrass, wild millet, green bristlegrass, and bottlegrass) develop dense competitive stands and have heavy seed production in spring sown crops. Green foxtail is a principal weed of corn, soybean, cereals, flax, canola, sugar beets, and pastures. The amount of damage to the crop depends on the density of the stand, time of emergence, and length of time the weed and crop are competing. Weed surveys for herbicide-resistant green foxtail have revealed that many of these plants exhibit some degree of herbicide resistance (Beckie, H. J., A. Legere, A. G. Thomas, L. T. Juras, and M. D. Devine. 1996 Survey of Herbicide-Resistant Wild Oat and Green Foxtail in Saskatchewan: Interim Report AAFC Report, 22 pp.). Therefore, biocontrol of these plants is highly desirable. However, at present for most of these weeds there are no known satisfactory biocontrol agents for control of green foxtail.
An important aspect in the development of a successful biological control agent is an effective delivery system. For biocontrol agents delivered onto target weeds by spraying, it is common for the erect top leaf to survive the attack due to the poor retention of the biocontrol agent on this portion of the plant. Thus, new methods of applying biocontrol agents are desired in the art. Further, traditional application methods such as run-off spraying are generally not suitable for treatment of large areas and thus there is a need in the art for methods to reduce the application volumes of biocontrol agents without reducing the efficacy of the biocontrol agent on the target weeds. To date, variable efficacy has been observed with biopesticide agents at reduced application volumes (Jones 1994, Smith and Bouse, 1981)
Previous attempts to control green foxtail weeds with biocontrol compositions have been relatively poor. In particular, it was noted in other studies that the top leaf of green foxtail consistently exhibited the least amount of disease development following biocontrol application, and reduced spray retention is speculated as a cause because of the erect leaf architecture of green foxtail weeds. Further, the surviving leaf often contributes to regrowth from the apical meristem, reducing the effectivity of the biocontrol agent. Other factors, such as but not limited to age (Green and Bailey 2000) and mineral nutrient content (Filippi and Prabhu 1998) of the leaves, may affect the susceptibility of green foxtail weeds to fungal pathogens.
In field crops, application volumes over 600 L/ha are considered high (Matthews, 1992), and the trend is generally toward volume reduction. In previous experiments, when applied at volumes between 100 to 800 L/ha, the agent 94-409A showed significantly lower efficacy in comparison to the runoff airbrush spray using the same spore concentration. Commonly the erect top leaf developed little disease and survived the attack. It is believed that the poorer efficacy is related to a lower amount of fungal propagules received and retained on the plant
It is an object of the present invention to overcome drawbacks of the prior art.
The above object is met by a combination of the features of the main claims. The sub claims disclose further advantageous embodiments of the invention.